Welcome to Science Buddies and thank you for your question. The fun stuff is done (i.e. the experiment), now comes the writing, often the hardest thing to get motivated about! As I learn almost every day in real life...it's not done until the paperwork is done!
The thing to remember about abstracts is it is a very short, normally only a couple hundred words or so, description of the project. It is designed to let the reader know what the project was, enough details about the proceedures to understand what the experiment was, what the hypothisis is and results. It should still include the following major categories just in a very brief form: Introduction, Problem Statement, Procedures, Results, and Conclusion. One of the main purposes of the abstract is to motivate the reader to be so interested in what you did that they want to read the full report and/or hear your presentation on the subject.
If you haven't been, Science Buddies has a project guide, located here: http://www.sciencebuddies.org/science-f ... ndex.shtml
which has a section on abstracts, with examples, located here: http://www.sciencebuddies.org/science-f ... ract.shtml
To get the writing juices flowing, I suggest writing about the project without regard to size. Sort of a free style writing, and then edit down into a concise description of the project within the size requirements, if there is one. Just pretend you are describing the project to a friend on the phone...You'll give enough detail so they understand, you'll try to keep it interesting enough that they won't hang up, and you'll keep it short by concentrating on key points without going into ALL the gory details so your arm doesn't fall asleep holding the phone.
I hope this helps.
"As the circle of light increases, so does the circumference of darkness around it."
~ Albert Einstein